This invention relates to methods and apparatus for retrieving information from an optical disk at high data rates by simultaneously and synchronously reading multiple adjacent tracks.
Due to their high storage density, long data retention life, and relatively low cost, optical disks are becoming increasingly popular as a means to distribute information. Large format disks have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and DVD, have become prevalent in the personal computer field, to the point that the DVD format may soon replace videotape as the distribution medium for video information.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, are encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data are read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the laser""s field of illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal which may be decoded to recover the digital data stored on the optical disk. The recovered digital data, which may include error correcting codes and additional subcoded information, are further processed to recover the stored data which may then be converted to audio signals, or used as executable programs and data depending on the type of optical disk being read.
To be able to retrieve data from anywhere on an optical disk, the optical systems include a pickup assembly which may be positioned to read data from any disk track. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity.
Because in most previously known systems the data are retrieved from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 xcexcm, while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of up to 16xc3x97 standard speed are commercially available, and even faster speeds have been achieved by moving to constant angular velocity designs. However higher disk rotational speeds place increasing demands on the optical and mechanical subsystems within the optical disk player, create greater vibration, and may make such players more difficult and expensive to design and manufacture.
Other previously known techniques for increasing average data transfer rates involve methods to intelligently anticipate future read requests by a host processor. It has been observed that data access by computers frequently exhibit xe2x80x9clocality of reference,xe2x80x9d which means that a future data access will be local, in either space or time, to a previous data access. Thus a CD-ROM drive or controller can xe2x80x9cread aheadxe2x80x9d and buffer the data that the host processor is likely to request next. When the host processor next requests data from the optical disk drive, the drive first checks if the requested data have already been read and buffered. If the data have already been buffered, the drive simply sends the buffered data to the host, avoiding the delays associated with repositioning the pickup assembly and reading data from the optical disk itself. While such caching techniques may speed up average data access times, the maximum data transfer rate is still limited by the rotational velocity of the optical disk within the optical disk reader.
Commonly assigned U.S. Pat. No. 5,627,805 describes a system to increase disk reading speeds by reading multiple tracks simultaneously. The data is read using a wide area reading beam, which is focussed onto a plurality of tracks on the disk. A detector comprising a matrix of photo-detector elements provides an image of the wide area, from which track signal data for each of a plurality of tracks is extracted by a virtual tracking system. Alternatively, the track data signals may be provided by a multi-beam optical pickup, as described in commonly assigned, copending U.S. patent application Ser. No. 08/804,105. The track data signals are then sampled, to produce a plurality of digital data streams, which are multiplexed into a single data stream before demodulation, decoding, and error correction.
Since the multiplexed data stream contains data from multiple tracks, which may be read from a disk that is spinning at multiple times the standard speed, the rate at which data in the multiplexed stream must be processed may be very high. For example, in a multi-beam CD-ROM reader that reads seven tracks simultaneously and spins the disk at 8xc3x97 the standard speed (giving the approximate equivalent of a 56xc3x97 drive), the data rate in the multiplexed data stream will be approximately 240 million bits per second in the demodulation stage (17 million words per second at 14 bits per word). By the time the data reaches the error correction stage, the required data rate will have dropped to approximately 79 million bits per second (9.9 million bytes per second). As can be seen, the rate at which data in such a system must be processed requires use of high performance devices to perform the functions at each of the stages in the processing chain.
The present application is directed to an improvement in the system described in the above-incorporated patent, wherein the multiplexer is moved to a later position in the processing chain, so that the plurality of data streams from multiple tracks on the disk may be demodulated, decoded, and error corrected before being multiplexed into a single data stream. Apparatus in accordance with the present invention may use a plurality of inexpensive, relatively low performance devices to perform the steps of demodulation, decoding, and error correction, while delivering throughput similar to that achieved by a high performance device performing these operations on a single multiplexed stream of data. Using the techniques of the present invention, it may be possible to construct a high performance system for processing the multiple data streams read from an optical disk, and which has a higher throughput than other similar systems, using standard low cost components.
It would therefore be desirable to provide apparatus and methods which permit simultaneous processing of data from multiple tracks in an optical disk reader.
It would also be desirable to provide demodulation, decoding, and error correction circuitry having higher throughput and at lower cost than previously known circuitry for processing the data from multiple tracks in an optical disk reader.
In view of the foregoing, it is an object of the present invention to provide apparatus and methods for simultaneous processing of data from multiple tracks in an optical disk reader.
It is a further object of this invention to provide demodulation, decoding, and error correction circuitry having higher throughput and faster time to market than previously known circuitry for processing the data from multiple tracks in an optical disk reader.
These and other objectives of the invention are accomplished by placing the multiplexer in a position late in the processing stream, so that the data from the multiple tracks remain separate through the processing stages of demodulation, decoding, and error correction. Implementation of this scheme requires providing multiple demodulation, decoding, and error correction units.
Alternatively, the multiplexer may be placed in the processing chain between the demodulation stage and the decoding stage, so that demodulation is performed in parallel on the multiple data streams, but decoding and error correction are performed on a single, multiplexed data stream. The multiplexer could also be placed in the processing stream between the decoding stage and the error correction stage, so that demodulation and decoding are performed in parallel on the multiple data streams, and error correction is performed on a single, multiplexed data stream.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.